I am the ultimate killing machine, used not once but TWICE, I can be made by humans. What am I?

Answer: War

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I am three simple words, everyone wants to hear. Three simple words, such simple words, and yet… life changing… What am I?
I am too much for one but not enough for two. Give me to a third and I am gone. What am I?
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I am what remains when nothing is left. What am I?
I am wood that is neither hard, straight, or crooked. What am I?